1. Field of the Invention
The present invention relates generally to an apparatus and method for controlling the structure of porous membranes or thin films utilizing a nonuniform electric field. More particularly, the apparatus and method may be used to reduce or eliminate macrovoid pore defects or to create a desired pore structure in polymeric membranes or thin films.
2. Description of Related Art
Both membranes and thin porous films are barrier layers that permit the passage of some particles, colloidal or molecular aggregates, molecules, or ions relative to others. Membranes are distinguished from thin films in that they are used either to separate or release at a controlled rate these particulate, colloidal, molecular, or ionic components. Typical applications of membranes include the production of fresh water from sea-water and the controlled release of drugs and pharmaceuticals via transdermal devices. Porous thin films are used in non-separations or non-controlled release applications in which it is desirable to pass one or more components through the film in one direction while preventing the passage of the components through the porous thin film in the opposite direction. Typical applications of thin porous films include breathable garments, surgical dressings, and screen-printing media.
Although membranes and porous thin films can be formed from ceramic and metallic materials, most commercial membranes and thin porous films are made from polymeric materials. Polymeric membranes and thin films are often produced by a phase-inversion casting process, in which a single phase polymer solution inverts into a two-phase dispersion consisting of a polymer-lean phase and a polymer-rich phase. The single-phase solution is formed from one or more polymers, one or more solvents for the polymer or polymers, and, possibly, one or more nonsolvents or other additives, such as plasticizers, surfactants, and nucleating agents. In phase-inversion casting processes, typically, the polymer-lean phase is initially dispersed in the polymer-rich phase. The polymer-rich phase then becomes the structural matrix of the membrane or porous thin film, and the polymer-lean phase becomes the pores. The phase inversion may be accomplished by wet casting, where the solution is immersed into a bath of one or more nonsolvents; by dry casting, where one or more solvents are evaporated from the dispersion; or by thermal casting, where the temperature is altered, causing the latent solvent or solvents to lose their solvent action for the polymer or polymers.
Membranes and thin films can also be prepared by interfacial polymerization processes, in which polymerization occurs at the interface between two immiscible liquids. When the two solutions come in contact with each other, polymerization begins at the interface and proceeds until a thin film is formed between the two phases. The polymerization reaction is limited by the mutual solubilities of the liquids, and the formation process is self-limiting. The polymeric layers are generally quite thin, often less than 1 micron in thickness.
During the fabrication of polymeric membranes or porous thin polymeric films by phase-inversion or interfacial polymerization processes, it is common for large pores, or macrovoids to form. Macrovoids typically have diameters of 10 to 50 microns, and they can extend a substantial distance through the thickness of a polymeric membrane or thin film. Usually, macrovoids are considered to be defects in polymeric membranes, because they reduce the useful volume of the membrane, cause pinholes that reduce the selectivity of the membrane, and can lead to structural failure in higher pressure separation applications.
However, macrovoid pores can also be useful in applications where large pores are desired. For example, uniform color intensity is obtained with printing media having pores that are uniformly distributed on the surface to which ink is applied, but which have a uniform depth and do not penetrate completely through the medium.
It is well known that macrovoids form in polymeric membranes when the casting process occurs very rapidly. In wet-casting, this occurs when the exchange of solvent and nonsolvent between the polymer solution and nonsolvent bath occurs very rapidly. In dry-casting, this occurs when the polymer solution contains a relatively large amount of nonsolvent or nonsolvents, and the solvent evaporates very rapidly. In thermal casting, this occurs when very rapid cooling causes a high degree of supersaturation in the polymer solution.
The size and distribution of pores can be controlled in a variety of ways. These include varying the composition of the initial casting solution from which the polymeric membrane or thin film is made; varying the initial thickness of the cast polymeric membrane or thin film; varying the composition of the ambient gas phase in the case of dry-casting or the nonsolvent immersion bath in the case of wet-casting; and adding nucleating agents, surfactants, plasticizing agents, or viscosity-enhancing agents to the polymer casting solution. For example, macrovoid formation can be reduced or eliminated by delaying phase-inversion, such as by including some solvent in the nonsolvent bath in wet-casting or by reducing the amount of nonsolvent in the polymer solution in dry-casting. Other ways to reduce or eliminate macrovoids include casting thinner membranes or films, increasing the viscosity of the polymer solution, and adding surfactants to the casting solution or nonsolvent bath.
However, these process alterations can cause other undesirable changes in the membrane or thin-film structure. In particular, decreasing the rate at which phase-inversion occurs can cause a thicker xe2x80x9cskinxe2x80x9d to form at the interface between the polymer solution and the nonsolvent bath in wet-casting or between the polymer solution and ambient gas phase in dry-casting. In some cases, a very thin skin layer, typically 0.01 to 0.1 microns, is desirable, since it provides a highly selective layer that can discriminate between different molecular species. Generally, a skin layer significantly thicker than 0.1 microns is undesirable, because it results in uneconomically low permeation rates.
It is an object of the present invention to provide an apparatus and method for controlling macrovoid formation in polymeric membranes and thin films.
It is a further object of the present invention to provide an apparatus and method for reducing or eliminating the formation of macrovoids in polymeric membranes and thin films, without altering the composition or thickness of the casting solution or the composition of the nonsolvent bath.
It is yet another object of the present invention to provide an apparatus and method for creating macrovoid pores of a desired size, depth, and or spatial distribution in polymeric membranes and thin films.
It is yet a further object of the present invention to provide an apparatus and method for controlling the size, depth, and spatial distribution of pores that are formed during processes for forming membranes and thin films, including phase-inversion processes, via a means that does not cause other undesirable effects such as thinner membranes or films, or less than optimal membrane or thin-film structure.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention broadly described herein, one embodiment of this invention comprises an apparatus for controlling the number and size of pores in a dielectric material formed by phase-inversion casting or interfacial polymerization from a precursor solution layer. In the casting process, the solution layer and the dielectric material each have a free surface side and a support side. The apparatus comprises at least one first electrode positionable on the free surface side of the dielectric material and a means for maintaining the first electrode at an electrical potential sufficient to provide an electric field through the precursor solution layer. The means for maintaining the first electrode at the electrical potential may comprise a battery or an ac or dc power supply, with one terminal connected to the first electrode. The other terminal of the battery or power supply may be connected to a second electrode on either side of the precursor solution, or the second terminal and the support side of the precursor solution may be connected to ground. Preferably, the dielectric material is a polymeric membrane or a polymeric thin film. The electrodes and the means for maintaining the electric field may be selected to control the size, depth, and number of pores in the dielectric material, or to decrease or eliminate the formation of macrovoid pores. The first electrode may be configured to provide a non-uniform electric field, such as by using a cylindrical rod or by attaching one or more conductive elements, such as rods, or by using a screen as the first electrode. If the apparatus is used for a wet-casting process, the portions of the first electrode and any attached conducting elements that are immersed in the nonsolvent bath should be electrically insulated.
In another embodiment, the present invention comprises an apparatus for phase-inversion casting or interfacial polymerization of dielectric materials. The apparatus includes means for supporting a precursor solution layer during a casting process, at least one first electrode positionable on the free surface side of the solution layer, and means for maintaining at least one first electrode at an electrical potential sufficient to provide the electric field. The apparatus may be adapted for use in a batch-wise or continuous casting process, and the process may include dry casting, wet casting, thermal casting, interfacial polymerization, combinations thereof, and modifications thereof. The means for supporting the precursor solution may comprise at least one material selected from metals, glasses, ceramics, microporous polymer films, webbing, fabrics, and composite materials, and it may be adapted for casting the dielectric material into a shape selected from flat sheets, pleated flat sheets, hollow fibers, and tubes. The apparatus may be adapted to use a non-uniform electric field to control the number, size, and/or depth of pores, including macrovoid pores, in the dielectric materials.
In yet another embodiment, the present invention comprises a method for controlling the number and size of pores in a dielectric material formed by phase-inversion casting from a precursor solution layer, wherein the solution layer and the dielectric material each have a free surface side and a support side. The method comprises the steps of positioning the precursor solution layer between a first electrode on the free surface side of the solution and a support for the cast material, providing an electric field between the first electrode and the support, and allowing the precursor solution layer to separate into two phases. The electric field may be provided with a power source, such as a battery or an ac or dc power supply, having one terminal in contact with the first electrode. Additionally, a second electrode may be provided on the support side of the solution layer, in electrical contact with the second terminal of the power supply. Alternatively, the second terminal and the apparatus may be grounded. The precursor solution may comprise a polymer or a polymer precursor; and the dielectric material may comprise a polymer membrane or polymer thin film. The first electrode may be configured as a screen, rod, or similar array of conductors, and one or more conductive elements may be attached to the first electrode and spaced apart from the precursor solution or dielectric material. The method may be used to control the number, size, and/or depth of pores, including macrovoid pores, in the dielectric material.
Still another embodiment of the present invention comprises a method for phase-inversion casting a dielectric material from a precursor solution layer, wherein the solution layer and the dielectric material each have a free surface side and a support side. The method comprises the steps of positioning the precursor solution layer between a first electrode on the free surface side of the solution and a support for the cast material, providing an electric field between the first electrode and the support, and allowing the precursor solution layer to separate into two phases. The method may be conducted in a continuous or batchwise manner, including dry-cast processes, wet-cast processes, thermal-cast processes, combinations thereof, and modifications thereof The method may also include the additional step of forming the dielectric material into a shape such as, but not limited to, a flat sheet, a pleated sheet, a hollow fiber, and a tube. The precursor solution may comprise a polymer or a polymer precursor, with the dielectric material comprising a polymeric membrane or a polymeric thin film.
Yet another embodiment of the present invention comprises a method for controlling the size and distribution of macrovoid pores in an interfacially polymerized dielectric material. The method comprising the steps of providing two precursor solution layers having an interface therebetween, providing an electric field through the solution layers and the interface, and allowing polymerization to occur at the interface. The dielectric material may comprise a polymeric membrane or a polymeric thin film. There may be an additional step of providing a support for the precursor solution layers and the dielectric material, with one of the precursor solutions between the support and the second precursor solution. The electric field may be configured to control the number, size, depth, and/or spatial distribution of macrovoid pores in the dielectric material. The method may be a continuous or a batchwise process, and it may include the additional step of forming the dielectric material into a shape selected from flat sheets, pleated flat sheets, hollow fibers, and tubes.